1. Field of the Invention
The present invention relates to a backlight apparatus for a liquid crystal display, which has at least two cold-cathode tubes.
2. Description of the Related Art
In recent years, the liquid crystal panels of liquid crystal display apparatuses have been enlarged in size. Further, the back surface of the liquid crystal panel is illuminated with plural (for example, two) cold-cathode tubes thereby to uniformalize the luminance of the screen of the enlarged liquid crystal panel. A lighting system (a backlight apparatus) shown in FIG. 4 is disclosed in JP-A-2000-124045 (on paragraphs [0016] and [0017], and in FIG. 5). The circuit of this backlight apparatus comprises a self-excited inverter 2, ballast capacitors C1 and C2, and cold-cathode tubes L11 and L12. Reference character T designates a step-up transformer, reference character W1 denotes a primary winding of the step-up transformer, reference character WF designates a feedback winding, reference character W2 denotes a secondary winding, reference character W2A designates a first part of the secondary winding, and reference character W2B denotes a second part of the secondary winding. The secondary winding W2 is grounded at the midpoint thereof. An intermediate tap of the primary winding W1 is connected to an input terminal IN through a choke coil L. Reference characters Q1 and Q2 designate push-pull connected transistors, reference character R denotes a bias-resistor, and reference character CC designates a resonance capacitor parallel-connected to the primary winding W1.
One OT1 of terminals of the second winding W2 is connected to one of terminals of the cold-cathode tubes L11 through the ballast capacitor C1. The other terminal of the cold-cathode tube L11 is grounded. Further, the other terminal OT2 of the secondary winding W2 is connected to one of terminals of the cold-cathode tube L12 through the ballast capacitor C2. The other terminal of the cold-cathode tube L12 is grounded.
When a DC voltage rectified by a rectifying circuit (not shown) is inputted to the input terminal IN, high frequency voltages, which differ in polarity from and are equal in value to each other, are generated at both the terminals OT1 and OT2 of the second winding W2. The ballast capacitors C1 and C2 limit electric currents flowing through the cold-cathode tubes L11 and L12, respectively, so as to change the electric currents into constant currents thereby to stabilize the lighting of the cold-cathode tubes L11 and L12. However, in a case where the electromagnetic coupling between the primary winding W1 and the secondary winding W2 is weak (or where the voltages produced at the terminals OT1 and OT2 are low), it is unnecessary to provide the ballast capacitors C1 and C2 in the circuit. Additionally, JP-A-2002-043148 (on FIG. 5) discloses a backlight apparatus similar to that shown in FIG. 4.